SPID#: 40 The primary physiologic regulator of platelet production, Mpl ligand, has recently been cloned and characterized. To define the regulatory role of Mpl ligand on platelet production and function we measured the effects of a recombinant truncated human Mpl ligand, megakaryocyte growth and development factor (rHu-MGDF) on megakaryocytopoiesis, platelet function and thrombogenesis in non-human primates. rHu-MGDF was administered to ten baboons for 28 days while performing pharmacokinteics and repeated measurements of 1) platelet count, volume, turnover and function ex vivo and in vitro; 2) marrow megakaryocyte number, volume, and ploidy; and 3) platelet deposition and fibrin accumulation on segments of vascular graft and endarterectomized aorta in vivo. Our results demonstrate that rHu- MGDF has the potential for achieving platelet hemostatic protection with minimal thrombo-occlusive risk. In related studies, we investigated the dose-response effects of pegylated rHu-MGDF (PEG-rHuMGDF) on platelet production and function. PEG-rHuMGDF is in a truncated polypeptide Mpl- ligand derivitized with poly-(ethylene glyco). In baboons, we demonstrated that PEG-rHuMGDF increases platelet production in a linear log-dose-dependent manner by stimulating megakaryocyte endoreduplication and new megakaryocyte formation from marrow hematopoietic progenitors. These findings suggest that appropriate dosing of PEG-rHuMGDF therapy during periods of chemotherapy-induced marrow suppression may maintain hemostatic concentrations of peripheral platelets without increasing the risk of thrombosis. Finally, in a rhesus monkey model of hepsulfam marrow suppression, both thrombocytopenia and neutropenia were abolished by initiating PEG-rHuMGDF therapy on day one and subsequently adding recombinant human granulocyte colony stimulating factor (rHu-GCSF) after one week for the remaining period of therapy. This study demonstrates the importance of optimizing the dose and schedule of cytokine combinations after severe myelosuppressive chemotherapy.